It is well known to spray apply cementitious slurries on to metal structural members to provide a heat resistant coating. A particularly successful type of product in this field is a gypsum-based formulation which contains, in addition to the gypsum binder, a lightweight aggregate, a fibrous substance. e.g. cellulose and an air entraining agent. See U.S. Pat. Nos. 3,719,513 and 3,839,059. When applying a heat resistant coating on a metal structural member, the slurries used are generally prepared at ground level and pumped to the point of application. The point of application can exceed 20 to 30 stories in high rise construction and the slurry is generally applied through a spray nozzle.
Slurries must possess a number of important properties to be suitable as heat resistant coatings and some examples of these important properties are set forth below. First, they must be sufficiently fluid to be pumped easily and to great heights. Second, they must retain a consistency sufficient to prevent segregation or settling of ingredients and provide an adequate "yield" or volume of applied fireproofing per weight of dry mix Third, they must adhere to the metal the structural member is comprised of, both in the slurried state and after setting. Fourth, the slurry must set without undue expansion or shrinkage which could result in the formation of cracks that can deter from the insulative value of the coating.
A further factor which affects the formulation of the hydraulic cementitious compositions is set time. Fireproofing mixes, such as W.R. Grace & Co.-Conn.'s MONOKOTE.RTM. brand materials, are transported to the building site as dry mixtures and in a mixer an appropriate amount of water is added to form a slurry. The slurry is pumped from temporary holding equipment to the point of application. The preparation and application processes may span many hours and thus the setting time of the mix is generally heavily retarded to provide an acceptable field pot life. If the accelerators are introduced in the mixer, problems arise if the operation is suspended, as premature set-up occurs. Set-up is the hardening of the slurry composition. Similarly, where the components are pumped over distances, premature set-up can occur prior to reaching the final destination.
Although the addition of retarders provides a mix which remains pumpable over many hours, this long set time is undesirable once the composition has been applied to the structure. If applied under hot, dry conditions, the mix may dehydrate before setting and yield a less than optimum final product. In cold temperatures, the mix may freeze before setting, while the preferred sequence is setting before freezing. Where multiple layers of fireproofing are to be applied, the first layer must set at least partially prior to application of the next layer. The long set time thus requires the sprayman to move on to another portion of the structure, only to return several hours later for application of the next layer. This results in an inefficient use of manpower.
Prior to spraying the viscous hydraulic slurry, accelerators are sometimes added to the slurry to decrease the total set-time. Mixing problems are encountered when the accelerators are added to the slurry since the slurry is a very viscous substance (in the case of a cementitious slurry, the apparent viscosity is about 13000 cp) and the accelerator is a low viscosity liquid (in the case of aluminum sulfate, the viscosity is about 3 cp). The viscous hydraulic slurry flows as a plug (i.e., plug flow) with minimal or no turbulence and it is difficult to obtain mixing between the viscous slurry plug and the low viscosity accelerator being introduced into the flow path of the slurry. This difficulty is compounded by the fact that only a small amount of low viscosity liquid is used with a relatively large amount of high viscosity slurry, and the fact that the flow rate of the slurry is such that its residence time in the distributor is only about 0.1 seconds. Various attempts have been made to mix the accelerator and slurry.
The accelerator has been injected directly into the middle of the slurry upstream from the air injector which is used to dispense the viscous slurry. The set time for the viscous slurry was not consistent throughout the entire spray pattern and therefore was unsatisfactory.